I think each xpac stands on its own. For what WoD attempted to offer which was improved quest flow, aesthetically pleasing and interesting dungeons, challenging raids with memorable fights, improved PvP experience and garrison interest, it was very successful. However, like Cata it had major content cuts - which are extremely extremely extremely disappointing to the fans of lore or even casuals looking for a new zone to run around in. The quest story was fantastic - but in end game did not and has not followed through conclusively except through the Legendary quest and Garrison questline, which while logical choices, there wasn't enough there.
Garrisons though. Followers once max level and max gear really have no purpose - which is just an awful awful way to reward the time spent putting into them. Granted for a time there was a strong reward system in place, which got nerfed which deflated the enthusiasm people actually using /enjoying the feature might have had for the sake of people uncompelled to do anything at all. Gradually garrisons became a steadily uncompelling irrelevant 'bonus' for everyone rather than a strong story telling end game reward and feature that enhanced pvp and pve.
Let's not mention the utter lack of customization and profession failures either. Taking the old profession system which didn't care when or how you got about your crafting and chaining it to hourly cooldowns for a unit rate of production was completely and utterly asinine. The reasoning behind this control scheme isn't known. Perhaps to weed out hacks, perhaps to stabilize prices on large servers. The economic effect was a huge kick in the pants for small servers however, where fewer players meant rarer materials, less production, less resources, high prices with marginal ability to pay, nevermind the natural discrepancies between professions themselves.
As for the lack of customization... even while I and many players knew there would be no racial architecture (huge let down - the generic horde-ally shit was old back in WRATH) what was worse was the pure irrelevance of faction colors and guards for exalted city reps and ebon blade. To work so hard to unlock something for two sticks, two banners and a partial complement of mismatched guards, half of which roam around endlessly and not at their stations was such a huge shit that I initially thought there had been a mistake or needed to reboot or then a server restart. When none of those things did anything I was horrified and stunned at the poor poor poor level of thought, planning and laziness that had gone into making something that should have been special and interesting but instead took a shit on it instead.
TL;DR:
Effectiveness of following through with core goals and changes: A-
End Game PvE: B+
End Game PvP: C+
Following through with making WoW a special and personal experience: A (for questing) C (for some endgame story) F (for skipped content, incomplete arcs and story) F for garrison endgame and experience; D
Following through with meaningful rewards for all alternative content, including garrisons: F
Overall: C ... It passes. Some things like raiding were quality. The game was successful in offering core changes. I'd like to think, like Cata, this had the potential to be the best expansion, offered too much, promised too much and dropped the ball in a big way on multiple levels with the little things that ACTUALLY mattered.